1. Field of the Invention
The present invention relates generally to a non-invasive cardiac monitor and, more particularly, to a method and apparatus for determining the cardiac performance of an individual by selecting a R-wave amplitude and a T-wave amplitude having greatest magnitudes from a plurality of electrocardiogram wave forms for determining a stroke volume value which can be on a continuous basis.
2. Description of the Prior Art
Various means have been developed in order to quantify the cardiac performance of an individual. Two parameters are commonly obtained to quantitatively measure the individual's cardiac performance.
The first of the parameters, referred to as stroke volume, is defined as the volume of blood pumped by the individual's heart in one heartbeat. The second parameter, referred to as cardiac output, is defined as the volume of blood pumped by the individual's heart in one minute. Cardiac output, thus, is the sum of the stroke volume over sixty seconds, and, may be derived from taking the sum of an individual's stroke volume over sixty seconds.
Presently, several different invasive methods are utilized in order to obtain values for these parameters. One such method, the Fick method, determines a value for the stroke volume by determining oxygen consumption of the individual and detecting changes in an individual's arterio-venous oxygen concentration levels. A second invasive method, the thermodilution, determines a value of cardiac output through an analysis of the changes of the temperature of a cold bolus injected into the individual's circulatory system. The cold bolus, having a temperature less than that of blood, causes a temporary decrease in the temperature of the individual's heart as the bolus enters the heart chambers. Once the bolus is pumped from the heart chambers and replaced by the higher temperature blood, the temperature of the heart recovers. By measuring the amount of time required for the heart temperature to recover, the volume of blood pumped during this period of time may be calculated, such calculated value being extrapolated to produce a value of the cardiac output in sixty seconds. Instead of a cold bolus, a dye maybe injected and the stroke volume is determined from the dilution of the dye.
The use of the invasive methods of determining the stroke volume and/or cardiac output of an individual are potentially dangerous, and are frequently unable to be performed as catheters must be inserted into the heart or other parts of the circulatory system to obtain the required information. Additionally, both the Fick method and the thermodilution and dye methods, in actuality, measure the average stroke volume and/or cardiac output of an individual by measuring the amount of blood circulated over a specific period of time and dividing the measured volume by the number of heartbeats of the individual's heart during that period of time. As is inherent in any average value, the average may differ substantially from that of a single value. In this instance, the actual stroke volume associated with a single heartbeat of the individual may differ from the average as the individual's stroke volume may fluctuate considerably from heartbeat to heartbeat, depending upon the activity performed by the individual.
Three non-invasive methods of determining stroke volume are alternatively used. The first such method, the Doppler-ultrasound method, determines a value for the stroke volume of an individual by calculating the Doppler effect upon an ultra-high frequency sound wave reflected from moving blood cells. The second method, the bioimpedance method, calculates the value for stroke volume of an individual by modulating a d.c. current by a measured blood pressure wave. The third method, the echocardiography method, calculates a value for the stroke volume based upon measurements of the size of the heart chamber of the individual.
While the Doppler-ultrasound and bioimpedance methods of determining stroke volume are non-invasive procedures and incur little risk to the patient, the methods cannot be utilized when performing certain medical procedures, such as open heart surgery. For instance, in order to perform the Doppler-ultrasound method, sensors must be positioned, and often repositioned, in the esophageal and sternal areas of the individual, and in order to perform the bioimpedance method, eight electrodes must be positioned in precise locations. And the third non-invasive method, the echocardiography method, is of limited usefulness as only intermittent visualization of the heart chamber of the individual is possible. Therefore, the echocardiography method is also precluded for use during certain medical procedures, such as open heart surgery.
Most recently, electrocardiogram waveform changes were utilized to determine cardiac functions. One such method is disclosed in my earlier U.S. Pat. No. 4,622,980. In this disclosure, an electrocardiogram waveform is separated into its components parts, the center spike or R-wave, the left-side sinusoidal P-wave, and the right-side sinusoidal T-wave. The electrocardiogram waveform is quantified by measuring the R-wave amplitude and the T-wave amplitude, and then calculating the ratio of the R-wave amplitude to the T-wave amplitude. The ratio is first calculated when the individual is at rest. The same ratio is then calculated subsequent to the application of a stress to the individual's cardiovascular system. The pre-stress ratio is then compared with the ratio calculated subsequent to the application of the stress. This new value is referred to as the stress index, S, and may be utilized to relate stressful events in terms of electrocardiogram waveform changes on a numerical scale.
A second method is disclosed in U.S. Pat. No. 3,572,321, to Bloomfield. In this disclosure, the R-wave amplitude and T-wave amplitude are measured, and a ratio of the two values is calculated. If the ratio is less than a certain value, a cardiac insufficiency is indicated. However, a typical electrocardiogram consists of numerous different waveforms, with a separate waveform corresponding to leads of twelve different electrodes attached at different locations on an individual's body. Because the magnitude of the ratio is dependent upon which waveform is selected, the indication of cardiac sufficiency or insufficiency is dependent upon which waveform is selected. The indication of cardiac sufficiency or insufficiency is then dependent, at least in part, upon which waveforms is selected. This method is claimed to have utility as a quick indicator of cardiac performance during mass screening procedures.